Courtesan of the Nation Arc is the 45th arc in the Gintama series. This arc introduces the arc's main antagonist Tokugawa Sada Sada, Rotten Maizou and the recurring antagonists, the Naraku and its leader Oboro. This arc also shows more of GIntoki's past including his teacher, Shouyou.
Gintoki was invited to Yoshiwara to meet Suzuran, Yoshiwara's most beautiful courtesan, to thank him personally for saving Yoshiwara. Gintoki's excitement to see Suzuran was blown away after seeing Suzuran as an old lady, shocked to find she's still working as a prostitute even in an old age. He soon learns that Suzuran was still waiting to see someone, a person she loved who made a promise to see her in the next full moon, wrapping each other's pinkie with a strand of hair, and Suzuran mistook Gintoki for the man and wrapped her strand of hair in his pinkie. Gintoki and Tsukuyo eventually decide to help Suzuran find that man (Gintoki teased Tsukuyo for doing the so, defending she wanted the man killed).
They discover that the man they were looking for was the former shogun and proceeded to enter the palace. Only through Kagura was they able to enter the heavily guarded palace; Soyo Hime, the sister of Shogun Shige Shige who also happens to reside in the palace, and Kagura knew each other (to everyone's shock), and Princess Soyo allowed them to enter the palace easily.
Inside the palace, the Yoruzuya once again meets Sasaki Isaburo and Imai Nobume, who are guarding the palace and the shogun because of certain incidents. Sasaki now keeps an eye on Gintoki and his friends, though not harming them since they're guests to the princess.
Characters in order of appearance